At present, limestone desulfurization process and ammonia-based desulfurization process are mainstream processes in the whole world for removing sulfur dioxide from gases. In the limestone desulfurization process, large quantities of waste water and gypsum residues are produced during desulfurization, and a lot of investment and operating costs are required to treat these waste water and waste residues. Also, in the limestone desulfurization process, while 1 ton of sulfur dioxide is removed, about 0.7 ton of carbon dioxide is produced synchronously. With ammonia-based desulfurization process, basically no waste water or waste residue is produced, and added ammonia desulfurizer is converted into a useful ammonium sulfate fertilizer, thus it is more environmentally friendly. However, the existing ammonia-based desulfurization process often has the problems, such as ammonia escape, aerosol formation, etc.
Chinese patents CN 1283346C and CN 1321723C disclose a process for removing SO2 from coal-fired flue gas by using ammonia as a removal agent, in which the SO2 concentration in the clean gas is less than 100 mg/Nm3. However, the amount of ammonia escaping in the clean gas can be up to 12 mg/Nm3.
Chinese Patent Conn. 100428979C discloses an ammonia-based desulfurization process with crystallization inside a tower and an apparatus thereof, wherein the desulfurization tower is designed to be of a multi-section structure, successively including an oxidation section, a crystallization section, a cooling absorption section, a main absorption section, and a dehydration-demisting section from bottom to top. In the process, the evaporating ability of flue gas is utilized for crystallization to reduce operation energy consumption, the SO2 concentration in the clean gas is less than 200 mg/Nm3, and the ammonia content in the clean gas can be as low as 3 mg/Nm3.
Chinese patent application No. CN 201710154157.3 discloses a method and a device for ammonia-based removal of sulfur oxides and dust from gas, wherein the device consists of a gas purification and removal system, an oxidation system, an ammonium sulfate post-processing system, an ammonia supply system and an auxiliary system, and uses a process of multipoint ammonia addition and multi-stage control, thereby significantly inhibiting ammonia escape and aerosol formation, and achieving efficient desulfurization and dedusting effects.
Chinese patent application No. CN 201610322999.0 discloses a pH-based automatically adjusting ammonia addition system, mainly including a control cabinet, an aqueous ammonia tank, a first aqueous ammonia pump, a second aqueous ammonia pump, a pressure transmitter, an electromagnetic flowmeter, an electric control valve and a pH transducer, wherein the control cabinet is respectively connected to the pressure transmitter, the electromagnetic flowmeter, the electric control valve, and the pH transducer; the control cabinet is connected to the first aqueous ammonia pump and the second aqueous ammonia pump; the inlet end of the first aqueous ammonia pump is connected to the aqueous ammonia tank and the outlet end of the first aqueous ammonia pump is respectively connected to an inlet of an ammonia addition chamber and an inlet of a circulating pump; the first aqueous ammonia pump is respectively connected to the pressure transmitter, the electromagnetic flowmeter, the electric control valve and the pH transducer; the inlet end of the second aqueous ammonia pump is connected to the aqueous ammonia tank and the outlet end of the second aqueous ammonia pump is respectively connected to the inlet of the ammonia addition chamber and the inlet of the circulating pump; the second aqueous ammonia pump is respectively connected to the pressure transmitter, the electromagnetic flowmeter, the electric control valve and the pH transducer; and the electric control valve is respectively connected to the inlet of the ammonia addition chamber and the inlet of the circulating pump.
An automatic ammonia-adding system with stable and reliable system operation, a high automation degree and a simple process and being applicable to an ammonia-based desulfurization device is still required to achieve automatic multipoint ammonia addition and multi-stage control in the ammonia-based desulfurization device, and inhibit ammonia escape and aerosol formation.
It would therefore be desirable to provide improved apparatus and methods for adding ammonia to a flue gas desulfurization system to overcome shortcomings in the prior art.